Biomass particles are generally difficult to fluidize, such that a denser and more homogeneous inert particle media (generally sand) is employed as a fluidization media to improve transport phenomena. Bench and pilot-scale continuous operation of a pyrolysis BFB has been demonstrated by Dynamotive. Char removal from the reactor can be an issue: if char is very fragile, its particle size will decrease within the bed by attrition. When char particle size reaches a critical particle value, it is entrained out of the fluidized bed reactor and it must be separated from the gas and recovered via a cyclone. Therefore, the disengagement region must be carefully designed to allow char particles to exit the reactor once they are sufficiently small. The main advantages of BFBs for pyrolysis applications include a uniform reaction temperature (minimizes the formation of cold/hot spots in the bed) and capability to operate the reactor continuously (continuous biomass feeding). On the other hand,

Fig. 11.3 Pyrolysis units global schemes: a Bubbling fluidized bed [41], b circulating fluidized bed [41] and c rotary drum reactor [51]. (Reprinted from Ref. [41], Copyright 2012, with permission from Elsevier; Reprinted from Ref. [51], with permission from Prof. Dr-Ing. Roman Weber) the main disadvantage of BFBs is that the volatile will be mixed with the inert fluidizing gas. Therefore, the bio-oil can be recovered, but the non-condensable gas is diluted such that it can hardly be used as a primary energy source. Thus, energy must be obtained from the solid char, which is significantly detrimental to the process profitability.